1. Field of the Invention
The present invention relates generally to an electronic stethoscope, and more particularly pertains to an electronic stethoscope which produces an audible output response having frequency components and characteristics which accurately match those of the initially detected auscultatory sounds.
2. Discussion of the Prior Art
The inherent promise embodied in an electronic stethoscope is that it will match the tonal qualities of a standard flexible tubing stethoscope while reducing background and tubing noise, increasing the user's comfort, removing the risk of transmitting ear canal infections, allowing tape recording and conferencing among multiple users and compensating for a user's hearing loss. However, past commercial attempts to produce an electronic stethoscope have received little enthusiasm from the medical community because these units had significantly different frequency response characteristics when compared to standard rubber tubing stethoscopes, and also introduced objectionable signal distortion and background noise. To gain medical acceptance, an electronic stethoscope should sound the same to physicians who are trained and practiced in recognizing audio cues which are transmitted by an auscultation instrument. This requirement to recreate the sound of a given standard stethoscope is a taxing one from an engineering point of view.
The realistic reproduction of clinically important audible signals pushes currently available audio equipment to the limit by requiring high level, distortion-free response down to the subsonic range. An electronic stethoscope should have a wide dynamic range with a low noise level to satisfy the wide dynamic range, keen discrimination, and high sensitivity of the human ear. The audible portion of the frequency spectrum of heart sounds has been reported to be 40-500 Hz, and for Korotkoff sounds, the range is 20-300 Hz. The greatest energy of these signals is contained in the lowest frequencies, with resting heart sound pressure levels reported to be 80 dB-SPL (0 dB-SPL at 0.0002 dyne/cm.sup.2) at 20 Hz.
Kimball, et al. U.S. Pat. No. 4,220,160 discloses, for example, an electronic stethoscope in which detected audible heart sounds at sonic and subsonic frequencies are detected and converted into somewhat corresponding electrical signals, which are then transposed in frequency to a range more easily detectable by the human ear. The new frequency range can also be suitable for transmission over conventional phone lines, for discrimination of low intensity or brief heart sounds, and for the display of the heart sounds on conventional visual recording devices such as, cardiographs, storage oscilloscopes, and chart recorders. The transposition of the heart sound frequency components involves the addition of a constant frequency component to all of the heart sound frequency components in such a manner as to preserve the characteristics of the heart sound frequency components. The transposer circuit employs a voltage multiplier circuit for multiplying the heart sound frequency components with a constant frequency component to provide sum and difference frequency components of the heart sounds. The sum and difference frequency components are then filtered to produce only the sum of the heart sounds, which is then amplified and presented to a conventional loudspeaker, headphone, audio system, phone line, tape recorder, or radio transmitter with suitable bandwidth, for aural interpretation. This electronic stethoscope is typical of other prior art electronic stethoscopes in that the amplifier is simply a high fidelity electrical amplifier which does not exactly reproduce at the user's ear the original sound signals from the patient, and also introduces distortions thereto in accordance with the transfer characteristics of the amplifier, an acoustic to electrical transducer, and an electrical to acoustic transducer.